Mainly two types of process have been described for the production of PLA; the first one consists in a direct polycondensation of lactic acid, as described for instance in JP patent 733861; such type of process is limited by the use of a solvent and the difficulty of removing water out of the reaction medium.
It is well known that in a second type of usual process for producing PLA starting from lactic acid, a significant loss of products, including lactic acid, lactide, oligomers of lactic acid and similar, occurs at each step of the global polymerization process; such steps may be summarized as follows: (i) oligomerization of lactic acids into oligomers, (ii) cyclization of the oligomers into lactide species, (iii) purification of lactide to obtain a suitable grade to start the last step which is the polymerization by ring opening of the purified lactide. Of course after this last step a devolatilization shall take place to recover the non-converted lactide.
Depending of the various processes for the production of PLA, it may be said that a lot of matters are lost all along the achievement of the process, so decreasing considerably the overall yield of said process. Indeed, if we take as a reference, a theoretical process without any recycling, the overall yield is as low as about 50% (molar).
It was therefore envisioned in the past to control certain partial recycling of streams issued from the step of evaporation of water from the starting lactic acid aqueous solution, from the step of oligomerization and from the step of the cyclization reaction; with such recycling operations, the overall yield has reached values as high as 75% (molar); however, even with those efforts which have been made and described to recover and finally recycle such type of lost product, this is not yet satisfactory to conduct an industrial process.
Therefore, there is a need for a process which enables not only to drastically reduce such loss of products, but mainly to incorporate in a global process the implementation of the recovery of the lost products and a process to convert such products to the starting monomer or its derivatives.
Therefore, the object of the present invention is to provide a process for the production of PLA wherein the recovery of the oligomers of lactic acid, lactide, and catalytic residues as well are implemented in order to treat and convert them into the starting monomer or its derivatives.
Another object is to recover the residues at each step of the process.
A further object is to treat the residues of lactic acid oligomers.
Another object is to implement the recovery and the recycle of the streams of lactic acid and water.
Another object is to provide a trans-esterification process to convert the oligomers residues with an alcohol.
Finally the process of the invention should also provide for treating the formed lactate compounds by hydrolysis in order to recover the alcohol and the lactic acid or its derivatives.
At least one of these objectives is met by the process of the invention.